The present invention relates to a method and to an arrangement for synchronising at least one local oscillator with a central time-generating unit. The local oscillator serves a so-called element included in a network, and the time-generating unit is included in a so-called main unit, which is also included in the network.
The frequency of the local oscillator can be controlled by periodic and automatic calibration.
It has long been known that nodes or elements belonging to a telecommunications network need to be synchronised with a common time reference. This is particularly important in respect of telecommunications networks that serve mobile telephones, where one and the same time reference must be used between different base stations in order for a user to be able to move without hindrance between the areas covered by said base stations during an ongoing communication.
It has become more and more usual in recent times to use in telecommunications networks transmission media and transport techniques that although novel with respect to telecommunications have been used earlier for the transmission of data, for instance.
The problem encountered when using new transport techniques is one of transferring synchronisation from the core of the network to its peripheral borders. The physical medium and the protocol used are not always suitable for the transference of synchronisation. A typical example in this respect is the use of Internet Protocol (IP) technique between base stations and Base Station Controllers (BSC) in mobile telecommunications networks.
Known solutions to these problems are found in the use of autonomous clocks or in clocks that are locked to an available navigational system, such as the Global Positioning System (GPS).
For reasons of cost, autonomous clocks are normally comprised of quartz oscillators. These clocks, however, require periodic manual calibration in order to be able to generate a time reference signal within set requirements.
It is also known that oscillators can be controlled automatically with respect to frequency without requiring manual work, by automatically assigning to the oscillator a correction value in accordance with a given periodicity. One example of oscillators that can be controlled automatically are voltage-controlled oscillators with which a voltage level determines the frequency of the oscillator. In the case of an oscillator of this kind, it is possible to calibrate and adjust the oscillator periodically and automatically, by controlling the voltage level in question. However, this requires a relevant correction value to be given, i.e. a voltage level that corresponds to the frequency to which the oscillator shall be set. Accordingly, the expression xe2x80x9cthe frequency can be controlled by periodic and automatic calibrationxe2x80x9d used in this description of the present invention refers to an oscillator that can be controlled frequency-wise by setting a signal, such as a voltage level, to a specific value in the absence of manual work.
It is not always possible to use the GPS, because it is not always possible to receive requisite signals, such as in the case of underground base station installations. The use of the GPS may also be impractical purely for cost reasons.
The problems of transferring a synchronising reference signal can be divided into two separate groups. Firstly, it may be necessary for an absolute time to be known, i.e. the time of day (ToD), and secondly a local oscillator may be required to oscillate at the same frequency as a specified reference frequency within a given error tolerance. The present invention relates mainly to this latter problem, in other words to the possibility of allowing the oscillators active in the network to oscillate at a common frequency.
Listed below are prior publications, which describe various solutions to problems that can occur within this particular technical field.
Publication EP-A1-0 838 916 teaches a method of synchronising a receiving node to the bit rate of incoming bits. A time stamp is allocated to a data packet arriving at a receiving buffer. When the data packet is read from the buffer, the time at which the data packet was read is compared with the time stamp and the time difference, i.e. the time between reading the data packet into the buffer and reading said data packet from said buffer provides a measurement of the number of data packets in the buffer.
The bit rate with respect to the receiving node is regulated in accordance with a desired number of data packets in the buffer. The bit rate is decreased when the number of data packets becomes too small and is increased when the number of data packets becomes too large. The local bit rate can be adjusted to be the same as the bit rate of the transmitting node in this way.
Publication U.S. Pat. No. 5,822,383 also describes a solution to the problem of synchronising a receiving node to the bit rate of a transmitting node.
Publication WO 98/13969 deals with the time of day problem. A good time reference is already found available through GPS signals to network nodes. Publications WO 98/13966 and SE-C2 508 460 also describe a solution in which all system nodes have access to a GPS signal.
In Proceedings: Twelfth Real-Time Systems Symposium, there is presented the article xe2x80x9cEfficient Synchronization of Clocks in a Distributed Systemxe2x80x9d by Sympath Rangarajan and Satish K. Tripathi, which describes the possibility of allocating different clocks in a system a common frequency that is based on a mean value of the different frequencies of the system clocks, by virtue of the different clocks in the system sending time stamps to one another and thereafter being synchronised to a common clock value calculated in accordance with a common statistical algorithm. The system clocks are synchronised in this way to a statistically calculated common frequency and not as slave clocks to a master clock.
A known method of synchronising a local oscillator with a central oscillator is to use time stamps, where a first time stamp is sent from the local oscillator to the central oscillator and a second time stamp is sent back in return. The information that is sent back includes information as to when the first time stamp was received and when the second time stamp was sent. The local oscillator can be synchronised with the central oscillator on the basis of the round-trip delay and on the basis of the information included in the second time stamp.
In the 1997 IEEE International Conference on Communications, there was presented an article xe2x80x9cOn Assessing Unidirectional Latencies in Packet-Switching Networksxe2x80x9d by Andreas Fasbender and Ingo Rulands, which describe the possibility of synchronising a local oscillator with a centrally positioned oscillator on the basis of round-trip delay.
This article also describes the possibility of using a one-way delay in ascertaining whether or not the communications path between two units in a network is suitable for transmitting time information that is reliable from the aspect of synchronisation.
It should also be mentioned that in a proposal to an American standard designated ANSI T1.101-1994 xe2x80x9cDraft American National Standard for Telecommunicationsxe2x80x94Synchronization Interface Standardxe2x80x9d, there are described and specified types of digital network interfaces that can transfer references for synchronisation and synchronisation specifications relating to reference signals in network interfaces between DS1 and SONET.
Section 8 of this publication describes how a local oscillator can be controlled to determine whether it maintains a specific quality standard for oscillators after a reference signal to the oscillator has been lost.
Oscillators can be divided into categories designated xe2x80x9cStratumxe2x80x9d, where Stratum levels, Stratum 1 to Stratum 4, denote the accuracy levels of the oscillators and where Stratum 1 denotes a highest level and Stratum 4 a lowest level. Each level can also be divided into sublevels in accordance with different alphabetical designations.
ANSI T1.101-1994 discloses computing algorithms for how relative frequency deviations and drifts of an oscillator can be computed and their respective limits with regard to the different Stratum levels.
Technical Problems
When considering the earlier state of the art as described above, it will be seen that in connection with a method or an arrangement where a local oscillator serves a so-called network element and where a time-generating unit is included in a so-called network main unit, and where the frequency of the local oscillator can be controlled by periodic and automatic calibration a problem resides in enabling the local oscillator to be synchronised with the central time-generating unit even when the network acting there between is unsuitable for or incapable of transferring the information required for direct synchronisation.
Another technical problem is to enable such information to be transferred solely by sending a periodic time stamp from the time-generating unit to the element concerned.
Another technical problem is one of providing a well-defined and repeatable transfer time for the transfer of a time stamp between the main unit and the element concerned.
Still another technical problem is one of sending between the main unit and the element concerned time stamps required for synchronisation, without interfering with other network traffic.
Another technical problem is one of enabling the arrival time of a time stamp at a receiving element to be safely evaluated statistically.
Yet another technical problem is one of finding a mathematical algorithm that enables calibration of a local oscillator with a starting point from measured intervals between the time stamps received by a network element.
Another technical problem is one of dividing a calibration process into different phases, where relevant time intervals are given for correct calibration and adjustment ment of the local oscillator.
Still another technical problem is one of adjusting a local oscillator when the signal from the time-generating unit is lost.
A further technical problem is one of adjusting a local oscillator in accordance with a received calibration value.
Yet another technical problem is one of evaluating a local oscillator that has been newly installed, during an initiation interval so as to be able to obtain quickly features that are characteristic of the newly installed local oscillator.
Another problem is one of being able to provide an element that includes a newly installed local oscillator with necessary information for enabling this element to avail itself of the advantages afforded by the invention.
A further technical problem resides in synchronising a local oscillator with respect to absolute time, i.e. ToD, in addition to synchronising a local oscillator with respect to frequency deviations and correcting the oscillator in this regard.
Another technical problem is one of being able to relieve respective elements from requisite calculations or computations and the hardware and software required to this end in connection with the calibration of a local oscillator.
Yet another technical problem is one of providing a suitable reference for the central time-generating unit.
Solution
With the intention of solving one or more of the aforesaid technical problems, the present invention takes its starting point from a method or an arrangement for synchronising at least one local oscillator with a central time-generating unit, where the local oscillator serves a so-called network element, where the time-generating unit is included in a so-called main unit also included in the network, and where the frequency of the local oscillator can be controlled by periodic and automatic calibration. That part of the network operative between the main unit and respective local elements is either unsuitable for or incapable of transferring information required for direct synchronisation of the local unit.
The invention relates to both a method and an arrangement. These are described separately in the following description of embodiments at present preferred while both the method and the arrangement are described more comprehensively and in common in this description of the solution.
With the intention of enabling the local oscillator to be synchronised with the main unit regardless of the deficiencies of the network operating there between, it is proposed in accordance with the invention that the time-generating unit sends a time stamp at pre-defined time intervals, that the physical conditions within that part of said network which includes said main unit and said element, here referred to as a delimited network, are so well defined that the transfer time of a time stamp from the main unit to respective elements is known to respective elements with a given degree of certainty, and that the mutual arrival times of received time stamps are used together with the known transfer time to calibrate the local oscillator.
A well-defined and repeatable transfer time between the main unit and respective elements can be provided by giving the time stamps the highest priority over all other information transmissions within the delimited network.
According to the present invention, the time stamps shall be transmitted to all elements within the delimited network in a general broadcast.
With the intention of enabling the arrival time of a time stamp to be guaranteed statistically, it is proposed in accordance with the invention that a group of time stamps are transmitted with each transmission within a short time interim of one another, such as three mutually sequential time stamps at one-second intervals.
The present invention provides a mathematical method in which the variation of the time stamp arrival times is used for said calibration, and where these variations are described in a time error which constitutes the difference between the pre-defined, and therewith expected, time interval and the actual time interval between received time stamps measured by the local oscillator.
This time error can be divided into an oscillator-dependent component and a network-dependent component, where the oscillator-dependent component is determined by the characteristic of the local oscillator and the network-dependent component is determined by the variations in transmission times from the time-generating unit to the element for respective time stamps and any possible measuring error in the measuring method used.
An evaluating period for calibrating a local oscillator is divided into a first phase and a second phase, where the first phase continues at least until the network-dependent component no longer has any deleterious affect on the calculation of the relative frequency deviation with respect to the local oscillator, and where the second phase is commenced immediately after the first phase.
The second phase includes continuous evaluation of the time error and is terminated with calibration and adjustment of the local oscillator.
According to the present invention, the second phase is terminated before the relative frequency deviation of the local oscillator has reached a maximum accepted relative frequency deviation.
Alternatively, the second phase may be terminated prior to the time error reaching a predetermined value that is smaller than or equal to a maximum accepted time error.
The mathematical method used includes an evaluation of the relative frequency deviation of the local oscillator in accordance with the formula   Y  =                    0        ⁢                  ,                ⁢        006                    N        ⁢                  xe2x80x83                ⁢                  τ          0                      ⁢                  ∑                  i          =          1                N            ⁢                        TE          i                ⁡                  (                                                    2                ⁢                i                                                              N                  2                                -                1                                      -                          1                              N                -                1                                              )                    
where Y is the relative frequency deviation, N is the number of time stamps received, xcfx840 is the time interval between transmitted time stamps, TEi is the time error in respect of received time stamps, and Nxcfx840 is thus the time duration of the second phase.
It is proposed that the time stamps are sent at intervals of 15 minutes to 4 hours, such as with a one-hour interval. Other intervals are possible, and part of the criteria that control the choice of this interval will be discussed in the following description of preferred embodiments.
With the intention of enabling the local oscillator to be corrected even when the time stamps are lost, it is proposed that the frequency drift of this local oscillator is evaluated and that said drift is used to correct the local oscillator when the time stamps from the central time-generating unit are lost.
According to the present invention, this drift can be evaluated by the following formula:   D  =                    0        ⁢                  ,                ⁢        06                    M        ⁢                  xe2x80x83                ⁢                  τ          1          2                      ⁢                  ∑                  j          =          1                M            ⁢                        TE          i                ⁡                  (                                                                                                                6                      ⁢                                              j                        2                                                                                                            M                        4                                            -                                              5                        ⁢                                                  M                          2                                                                    +                      4                                                        -                                                            6                      ⁢                      j                                                                                      M                        3                                            -                                              M                        2                                            -                                              4                        ⁢                        M                                            +                      4                                                        +                                                                                                      1                                                            M                      2                                        -                                          3                      ⁢                      M                                        +                    2                                                                                )                    
where D is the drift of the local oscillator, M is the number of time stamps, xcfx841 is the time interval between transmitted time stamps, and TEj is the time error of respective time stamps.
The results of different calibrations may be stored, thereby obtaining a characteristic value of the drift of the local oscillator for use in adjusting the oscillator in the event of the time stamps from the central time-generating unit being lost.
With the intention of enabling a newly installed local oscillator to be evaluated, it is proposed in accordance with the invention that there is used during an initiation interval for the new local oscillator a time interval between transmitted time stamps that is much shorter than the time interval between transmitted time stamps in a calibrating process.
This time interval may be 10 seconds and the initiating interval may be 30 minutes.
According to the invention, these time stamps can be ordered through a request from the element that includes the new oscillator to the main unit stating a desired time interval, desired initiating interval, etc.
When initiating a new oscillator, it is possible to allow the main unit to send necessary parameters to the element that includes the new oscillator, such as anticipated time intervals between transmitted time stamps, the time duration of the first phase, the time duration of the second phase, and possibly other parameters, so as to enable the local oscillator to partake of synchronisation according to the present invention.
With the intention of subjecting the delimited network to the smallest possible load, it is proposed in accordance with the invention that the information part of the time stamps includes solely the information required to recognise a time stamp and any information in order to ensure transmission of the time stamp over the delimited network.
It is also possible to allow the time stamps to include information relating to absolute time, whereby determination of the Time of Day (ToD) is enabled.
In order to achieve a high degree of accuracy in determining ToD, the present invention may be combined with any method that functions in accordance with the Round Trip Delay principle, such as Network Time Protocol.
With the intention of relieving respective elements from the load required when calculating the frequency deviation of the local oscillator, it is proposed in accordance with the invention that respective elements collect requisite calibration information during the second phase, and that these elements send this information to the main unit and that the main unit performs the calculations, or computations, necessary to obtain a value for calibrating the local oscillator, and that the main unit sends to the element concerned a local oscillator adjustment value, and that this element performs the necessary adjustment.
According to the present invention, the time-generating unit may include a GPS receiver whereby the time stamp time reference is obtained.
Advantages
Those advantages that are primarily characteristic of a method and an arrangement according to the invention reside in enabling local oscillators to be synchronised with a main unit over a network regardless of the properties of said network. For instance, this enables a network constructed for IP (Internet Protocol), which is not suitable for direct transmission of synchronisation information, to be used for mobile telecommunication, such as GSM, which requires good synchronisation between system base stations.
The primary characteristic features of a method according to the present invention are set forth in the characterising clause of the accompanying claim 1, while the main characteristic features of an inventive arrangement are set forth in the characterising clause of the accompanying claim 23.